Although turbo compressors using a centrifugal compressor have advantages such as having better efficiency, being lighter in weight and having more stable in operation than reciprocating compressors, their allowable operating range (i.e., the flow rate range of a centrifugal compressor) is limited.
At a small flow-rate operating point of a centrifugal compressor (i.e., when the flow rate of a compressor is small), instable phenomena such as considerable fluid separation at the internal flow field occur, thus causing stall and accordingly surge. As a result, rapid decrease in the efficiency and the pressure-ratio of the compressor is caused, the life of the compressor is shortened, and accordingly the compressor is damaged in a short time. To cope with this, various countermeasures are taken to suppress instable phenomena such as stall of a compressor for an extended stable operating range.
For instance, for an extended stable operating range, a casing treatment for centrifugal compressor is used. The following Patent Documents 1 to 5 disclose a casing treatment, for example.
As a casing treatment in Patent Literetures 1 to 5, at an inner face of a casing surrounding an impeller of a centrifugal compressor are formed (or defined) an annular inlet that is downstream of a leading edge of the impeller and an annular outlet that is upstream of the leading edge of the impeller. With this configuration, when the inflow rate into the centrifugal compressor is small, the fluid is returned from the annular inlet to the annular outlet via a casing interior, whereby the apparent inflow rate into the impeller is increased. As a result, instable phenomena such as stall can be suppressed to extend a stable operating range of a centrifugal compressor.